The long-term objectives of this program include the laboratory synthesis of complex natural products possessing desirable biological activity as well as the development of new synthetic methodology which will simplify this task. Specific natural products to be studied include the promising antitumor agent rhizoxin, structurally simplified analogues of rhizoxin, and the highly cytotoxic marine natural product swinholide, as well as nisakinolide, and scytophycin C, both of which are structurally very similar to swinholide. The main thrust of the program is to develop new catalytic procedures for carbon-carbon bond formation using chiral Lewis acids to promote the union of two pi systems. Two new vicinal sp3 carbon centers can be generated in such reactions, and the aim is to control stereochemistry at these sites in both a relative and absolute sense. Specific examples which will be studied represent reactions between aldehydes and allylstannanes or allylsilanes spanning a wide range of structural types; these reactions represent catalytic alternatives to the most powerful enantioselective synthetic reactions presently available.